As part of biomass energy utilization, attempts are made to obtain ethanol by decomposition of cellulose or hemicellulose which is a principal component of plants. In such attempts, there are planned to use the obtained ethanol for fuel by partly mixing it mainly in automotive fuel or by using it as an alternative fuel for gasoline.
Principal components of plants include cellulose (polymer of glucose which is C6 monosaccharide composed of six carbons), hemicellulose (polymer of C5 monosaccharide composed of five carbons and C6 monosaccharide), lignin, and starch, and ethanol is generated by fermentation action of microorganisms such as yeast from saccharides such as C5 monosaccharide, C6 monosaccharide and oligosaccharide which is a complex of the monosaccharides.
For decomposition of cellulosic biomass such as cellulose or hemicellulose, the following three methods are planned to be industrially applied: 1) method of hydrolyzing by means of the oxidizing power of strong acid such as sulfuric acid, 2) method of enzymatically decomposing, and 3) method of utilizing the oxidizing power of supercritical water or subcritical water. However, in the acid decomposition method of 1), the added acid is an inhibitory substance for fermentation of yeast, and hence it is necessary to conduct a treatment of neutralizing the added acid before alcohol fermentation of saccharides after decomposition of cellulose or hemicellulose into saccharides, and this leads to difficulty in practical application for the economical reason in terms of the treatment cost. The enzymatic decomposition method 2) allows a treatment at a normal temperature and a constant pressure, however, an effective enzyme has not been found, and even if an effective enzyme is found, the production cost of the enzyme is probably high, and the prospects for industrial scale realization are still far from certain in the economical aspect.
As the method of hydrolyzing cellulosic biomass by supercritical water or subcritical water to produce saccharides of 3), Patent document 1 discloses a production method of saccharides capable of separating saccharides containing C5 monosaccharide and C6 monosaccharide from saccharides containing C6 monosaccharide and recovering them, in addition to obtaining saccharides from wood biomass with high yield and high efficiency. The production method of saccharides of Patent document 1 includes: a first slurry heating step (S1) of heating a slurry prepared by adding high-temperature, high-pressure water to wood biomass; a first separating step (S2) of separating the heated slurry into a liquid ingredient and a solid ingredient; a second slurry heating step (S3) of adding water to the separated solid ingredient to render it a slurry, and heating the slurry; a second separating step (S4) of separating the heated slurry into a liquid ingredient and a solid ingredient; and a useful ingredient acquiring step (S5) of removing water from the separated liquid ingredient to acquire saccharides; and is characterized by further acquiring saccharides by removing water from the liquid ingredient separated in the first separating step (S2) in addition to acquiring saccharides in the useful ingredient acquiring step (S5).
Patent document 2 discloses a method of hydrolyzing biomass that hydrolyzes biomass by the use of pressurized hot water, comprising: a first step of hydrolyzing mainly hemicellulose in the biomass; and a second step of hydrolyzing mainly cellulose in a residue obtained in the first step, wherein a liquid used in the first step includes a filtrate obtained by solid-liquid separation conducted after end of the second step. Patent document 2 also discloses to use, as a liquid for use in the hydrolysis of the first step, part of water that is recovered after washing a residue obtained by solid-liquid separation after end of the first step with water, together with the filtrate obtained by solid-liquid separation after end of the second step, and to use the remainder of the slurry in the second step.
On the other hand, a method of continuously separating and washing a solid in a suspension by using a plurality of thickeners is known. Patent document 3 discloses a multi-stage thickener washing method for washing red mud by arranging a plurality of thickeners for washing in series, wherein part of over flow water of at least one thickener for washing is added to a slurry supplied to the thickener. Similarly, Patent document 4 discloses a method of adding a red mud sedimentation auxiliary agent in sedimentation of red mud, wherein in sedimenting red mud from a red mud slurry generated in the Bayer process in a sedimentation step operated under normal pressure and/or under increased pressure, a red mud sedimentation auxiliary agent is preliminarily added and diluted in a circulating clear liquid and/or washing water of the sedimentation step, and introduced into the red mud slurry to be treated. Patent document 4 also discloses washing red mud by using a plurality of thickeners (FIG. 4).